Field of the Invention
The invention lies in the field of communications. The invention relates to an acquisition method in which a received, coded spread sequence having a length of m bits is correlated with a locally generated spread sequence m times, in each case delayed by one bit, in order to form m correlation responses. The invention also relates to a configuration for carrying out the method.
One fundamental spread method is the direct sequence method, in which, before transmission, a message is stretched, bit by bit, to a great extent over time and is modulated with a pseudo-random binary sequence. If the receiver knows the binary sequence, the receiver can extract the message from the pseudo-random noise signal produced.
Such methods are used in data communication, position finding and navigation. A problem that arises in such applications is that a receiver receives signals from a number of transmitters, and the time relationship of the signals must then be determined in order to identify the signals.
One important field of application for such a method is, in particular, real-time satellite navigation using the NAVSTAR GPS system (Navigation System With Timing And Ranging, Global Positioning System). For such a system, a large number of satellites are used as a transmitter network, and are controlled from ground stations or monitoring stations. At least four satellites must be in a receiver""s field of view in order to find the position of the receiver in three dimensions. The messages from the transmitters are decrypted in the receiver to allow the necessary calculations to be carried out based upon them. For a receiver to be able to identify a satellite and to be able to evaluate its information, the receiver must know a transmitter-specific code (gold code), which is transmitted as a periodic signal sequence of predetermined length. In order to find the code, all the codes of the satellites are stored in the receiver. The code of a random satellite is then normally produced first of all, and is compared with the received code. If the code is not the expected satellitexe2x80x94as is normally the casexe2x80x94comparisons with the stored codes must be carried out until a match is found.
Furthermore, because the phase of each arriving signal is unknown, the signal with the received code must be shifted until a possible match is found. The comparison is carried out through the correlation function of the signal, which becomes virtually 1023 when synchronization occurs. If one considers all possible variants for such a search process, it is necessary, in principle, to accept long search times.
A search grid with 1023 search cells must be checked for a match with the gold-code spread sequence of 1023 bits, with a time interval equal to one bit length being specified between two search cells. The check results in a resolution of xc2xd a bit length. With optimum synchronization, the sampling time is chosen to obtain the maximum correlation response amplitude (see FIG. 2). The selection also results in the maximum signal-to-noise ratio. In the worst case, the signal power may be reduced down to one quarter of this level.
A prior art method for reducing the power reduction is to refine the search grid by reducing the time interval between search positions to a half or a quarter. However, the number of search processes must also be doubled or quadrupled so that the acquisition time is considerably lengthened.
It is accordingly an object of the invention to provide an acquisition method and configuration for carrying out the method that overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and that reduces the acquisition time.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a communication acquisition method, including the steps of forming m correlation responses by correlating a received, coded spread sequence having a length of m bits with a locally generated spread sequence m times, respectively delayed by one bit, forming a correlation result by delaying each correlation response and adding each correlation response to a succeeding correlation response, the delay being an integer multiple i of m and of a further bit, where i=0, 1, 2, 3, . . . , and carrying out a maximum search over all m correlation results.
In accordance with another mode of the invention, the locally generated spread sequence is a section of a transmitted spread sequence.
With the objects of the invention in view, there is also provided a configuration for performing a communications acquisition, including an adder having a first adder input, a second adder input, and an adder output for outputting a correlation result, a delay device having a delay input, a delay output connected to the second adder input, and a delay being an integer multiple i of m and of a further bit, where i=0, 1, 2, 3, . . . , a correlator having a correlator output connected to the first adder input and to the delay input, the correlator configured to receive a coded spread sequence having a length of m bits and to correlate the coded spread sequence with a locally generated spread sequence m times, in each case delayed by one bit, the correlator outputting m correlation responses at the correlator output, and a search unit connected to the adder output and configured to carry out a maximum search over all m correlation results.
In accordance with a concomitant feature of the invention, the adder, the delay device, the correlator, and the search unit are adders, delay devices, correlators, and search units, respectively.
Reduction of the acquisition time is achieved in that each correlation response is delayed and is added to a succeeding correlation response to form a correlation result, in that the delay is an integer multiple i (i=0, 1, 2 . . . ) of m and of a further bit, and in that a maximum search is carried out over all m correlation results. If i greater than 0, the method guarantees optimum results only for spread sequences that are not modulated with data.
One basic idea of the invention is, accordingly, for two adjacent search cells to be linked to one another. The result of such linking is that the correlation result has a broadened curve shape in the critical upper region in comparison with a conventional curve shape. The broadening amounts to one bit length, and the broadening does not result in any reduction in signal power. As such, the amplitude of the correlation response is independent of exact synchronization of the sampling time and of the search grid. At the same time, the signal-to-noise ratio is considerably improved by comparison with the worst case described above.
A preferred development of the invention is for the locally generated spread sequence to be a section of the transmitted spread sequence. This results in a further improvement of the acquisition time, particularly with long spread sequences.
For the configuration, reduction of the acquisition time is achieved by simple circuitry where the output of a correlator is connected to a first input of an adder, and where the output of the correlator is connected to a second input of the adder through a delay element. A maximum search unit is adapted to carry out a maximum search over all the correlation results.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an acquisition method and configuration for carrying out the method, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.